Coupled Nitrogen and Calcium Cycles in Forests of the Oregon Coast Range
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Ecosystems (2006) 9: 63–74 DOI: 10.1007/s10021-004-0039-5 Coupled Nitrogen and Calcium Cycles in Forests of the Oregon Coast Range Steven S. Perakis,1,2* Douglas A. Maguire,2 Thomas D. Bullen,3 Kermit Cromack,2 Richard H. Waring,2 and James R. Boyle2 1US Geological Survey, Forest and Rangeland Ecosystem Science Center, Corvallis, Oregon 97331, USA; 2Department of Forest Science, Oregon State University, Corvallis, Oregon 97333, USA; 3US Geological Survey, Branch of Regional Research, Water Resources Discipline, Menlo Park, California 94025, USA ABSTRACT Nitrogen (N) is a critical limiting nutrient that concentrations are highly sensitive to variations in regulates plant productivity and the cycling of soil Ca across our sites. Natural abundance calcium other essential elements in forests. We measured isotopes (d44Ca) in exchangeable and acid leach- foliar and soil nutrients in 22 young Douglas-fir able pools of surface soil measured at a single site stands in the Oregon Coast Range to examine showed 1 per mil depletion relative to deep soil, patterns of nutrient availability across a gradient of suggesting strong Ca recycling to meet tree de- N-poor to N-rich soils. N in surface mineral soil mands. Overall, the biogeochemical response of ranged from 0.15 to 1.05% N, and was positively these Douglas-fir forests to gradients in soil N is related to a doubling of foliar N across sites. Foliar N similar to changes associated with chronic N in half of the sites exceeded 1.4% N, which is deposition in more polluted temperate regions, and considered above the threshold of N-limitation in raises the possibility that Ca may be deficient on coastal Oregon Douglas-fir. Available nitrate in- excessively N-rich sites. We conclude that wide creased five-fold across this gradient, whereas gradients in soil N can drive non-linear changes in exchangeable magnesium (Mg) and calcium (Ca) base-cation biogeochemistry, particularly as forests in soils declined, suggesting that nitrate leaching cross a threshold from N-limitation to N-saturation. influences base cation availability more than soil The most acute changes may occur in forests where parent material across our sites. Natural abundance base cations are derived principally from atmo- strontium isotopes (87Sr/86Sr) of a single site indi- spheric inputs. cated that 97% of available base cations can origi- nate from atmospheric inputs of marine aerosols, Key words: nitrogen cycle; N-saturation; base with negligible contributions from weathering. cations; calcium; magnesium; strontium; stable Low annual inputs of Ca relative to Douglas-fir isotopes; atmospheric deposition; marine seasalt growth requirements may explain why foliar Ca aerosols; nutrient limitation; Douglas-fir. INTRODUCTION actions that impact the long-term health and sta- bility of forest ecosystems. Where plant growth is Biogeochemical couplings between nitrogen (N) limited by N availability, moderate increases in N and calcium (Ca) are an important suite of inter- supply can stimulate both plant growth and Ca uptake from soil. More dramatic increases in N Received 23 March 2004; accepted 10 November 2004; published online 30 January 2006. supply can overcome N limitation, drive forests *Corresponding author; e-mail: [email protected] towards N saturation, and accelerate coupled 63 64 S. S. Perakis and others leaching of N and Ca. In this way, long-term in- examine the long-term consequences of elevated N creases in N supply have a dual effect of stimulating on forest biogeochemistry. We here use a range of plant Ca demands and depleting Ca from soils. Douglas-fir forests in the Oregon Coast Range to These opposing drivers are thought to cause pro- ask the following questions: Do variations in soil N gressively non-linear N–Ca interactions along gra- status translate to changes in N availability for dients of ecosystem N status, yet such processes vegetation? How does base cation availability, in rarely are represented in forest biogeochemical particular Ca and Mg, change across a wide gradi- models (McNulty and others 1996; Asner and ent in soil N? Are N–Ca interactions linear or non- others 2001). linear across variations in N status? At a single There is broad recognition that chronic N depo- study site, we also use natural abundance 87Sr/86Sr sition is causing many temperate forests to cross a stable isotope ratios to identify long-term ecosys- critical threshold from N-limitation to N-saturation, tem sources of base cations, and introduce the use with serious implications for plant-soil Ca cycling of novel 44Ca/40Ca stable isotopes to evaluate (Aber and others 1998; McLaughlin and Wimmer pathways of Ca recycling in the plant-soil system. 1999). At the same time that N supply has in- creased, atmospheric Ca inputs have decreased in METHODS many areas (Hedin and others 1994). Preferential depletion of Ca relative to other elements by forest Site Description harvest is also of particular concern, especially in We conducted this research in young Douglas-fir N-rich areas (Federer and others 1989). These forests in the north-central Coast Range of Oregon. changes, combined with long-standing impacts Results from two collections are reported: (1) foliar from acid deposition, are modifying ecosystem Ca and soil nutrient concentrations across 22 sites balances and creating Ca deficiencies over large spanning a gradient of soil N availability, and (2) areas of temperate forest (Shortle and Smith 1988; strontium and calcium isotope analyses from a Lawrence and others 1997; Adams and others single stand with high N availability. All stands 2000; Huntington 2000; Driscoll and others 2001; studied were established as plantations, and none Bailey and others 2004). It remains unclear from have been fertilized. Climate across the study area these studies, however, whether changes in eco- is temperate and maritime, with wet cool winters, system Ca cycling can be traced primarily to effects dry warm summers, and approximately 170 cm of excess N alone, and whether such changes will annual precipitation. intensify as long-term N inputs increase site N The 22 stands representing the N gradient were status. selected from a set being monitored to examine the Forests of the Pacific Northwest are far less im- effect of Swiss needle cast on Douglas-fir growth pacted by chronic N and other air pollutants than (Maguire and others 2002). Stands ranged from 11 many other temperate regions. These ecosystems to 25 years of age, and exhibited a range of needle are often considered N-poor and base-rich (for longevity from 0.8 to 3.6 years (Table 1). Soils are example, Sollins and others 1980; Cole and Gessel derived from either sedimentary (that is, sand- 1992; Fenn and others 2003). However, high rates stones and siltstones) or basalt parent material, of N input can occur in forests of the Pacific with Andic properties and silty to silty clay loam Northwest via biological N fixation associated with textures (NRCS, in press). Most study sites were red alder (100–200 kg N ha)1 y)1, Binkley and relatively flat and located on broad upland topo- others 1994). Fixed N contributes to exceptionally graphy, though a few sites may have received up- high soil N capital (up to 30,000 kg N ha)1, land colluvial inputs. The stand sampled for Remillard 2000), and can accelerate N cycling, strontium and calcium isotopes supported 45-year- nitrification, and Ca leaching in a manner similar to old Douglas-fir along a ridgeline with sandstone- chronic N pollution (Van Miegroet and Cole 1984). derived soils. These factors are thought to predispose coastal Douglas-fir forests to intensification of an endemic Sample Collection and Analysis fungal needle cast disease, resulting in 22–53% reductions in tree growth over the past decade, Samples of foliage and soil were collected across the particularly on N-rich sites (Waring and others N gradient in spring 2000. Sampling was based on 2000; Maguire and others 2002; El-Hajj and others five randomly selected target trees per site. Mineral 2004). soil (0–10 cm) at two locations within 5 m of each Exceptionally wide gradients of soil N in the target tree was collected using a 2 cm diameter Pacific Northwest provide a unique opportunity to corer, and all samples were combined to yield one Coupled Nitrogen and Calcium Cycles 65 Table 1. Site and Stand Characteristics of the Twenty-two Sites at the Time of Sampling in 2000 Site Mean Standing Annual Soil suborder Elevation Slope Stand age index needle volume incrementà Site (USDA)* Aspect (m) (%) (y) (m) age– (y) (m3/ha) (m3 ha)1y)1) Sedimentary Music road Alic Hapludand N 484 5 16 47 3.3 38 11.0 7 Andic Dystrudept N 278 50 25 43 3.5 141 18.0 5 Andic Dystrudept SW 322 55 23 40 3.0 176 23.7 77 Andic Dystrudept W 235 10 22 45 3.0 221 25.9 58 Andic Dystrudept E 80 15 17 45 3.1 102 22.1 85 Typic Fulvudand SW 121 25 12 38 1.9 211 11.3 Juno Hill Andic Dystrudept S 86 10 18 36 1.1 61 4.5 16 Andic Dystrudept SW 115 15 20 38 1.7 89 10.5 Smith Creek Typic Fulvudand S 167 20 17 34 1.9 77 16.3 39 Andic Dystrudept S 23 10 21 45 1.8 123 16.3 Cloverdale Typic Fulvudand E 105 10 16 30 0.8 129 4.2 Basalt Steinburger Andic Dystrudept E 539 22 – 42 2.9 62 16.1 Jensen Typic Palehumult W 361 25 11 42 3.4 24 7.8 Hoag pass Alic Hapludand NW 755 20 12 30 3.6 27 60 Typic Fulvudand N 201 75 22 40 3.2 130 21.6 East Beaver Typic Fulvudand S 127 57 20 36 1.8 137 13.9 108 Alic Hapludand W 269 40 16 42 2.9 98 16.7 Chopping block Andic Dystrudept W 76 5 19 35 1.7 88 8.4 Powerline road Typic Fulvudand NW 78 47 17 36 1.8 92 15.7 51 Typic Fulvudand N 171 85 16 32 1.5 9 1.7 101 Typic Fulvudand SW 262 15 24 31 1.8 31 3.3 119 Typic Fulvudand S 366 30 18 13 0.9 2 0.5 Divided into sedimentary versus basalt soil parent material, ordered by increasing soil % N.